addListener method
Implementation
void addListener(Rx<T> rx) {
// if the current observable is not in the subscriptions
if (!_subscriptions.containsKey(rx)) {
// create a Subscription for this observable
final StreamSubscription subs = rx.stream.listen(subject.controller.add);
/// get the subscriptions for this Rx and add the new subscription
final listSubscriptions = _subscriptions[rx] ?? [];
listSubscriptions.add(subs);
_subscriptions[rx] = listSubscriptions;
}
}